1. Field of the Invention
The present invention relates to a digital signal reproducing apparatus for reproducing a digital signal recorded on an optical recording medium.
2. Description of Background Information
FIG. 1 schematically shows an exemplary configuration of a prior art digital signal reproducing apparatus, and FIG. 2 shows waveforms representing the operation performed by the circuits in the configuration shown in FIG. 1.
Referring specifically to FIG. 1, an optical pickup 1 irradiates an optical disc 3 as an optical recording medium, rotated by a spindle motor 2, with a laser beam. The optical pickup 1 also opto-electrically transforms reflected light from the optical disc 3 to produce a read signal "a" which is supplied to a comparator 4.
On a recording track on the optical disc 3, a plurality of recording pits Pt corresponding to digitally recorded signals as recording information are formed as shown in FIG. 2. A run length NT of the recording pits Pt is classified into several groups, one of which is employed in accordance with a particular modulation scheme for the digitally recorded signal. For example, if a digitally recorded signal is encoded in accordance with an RLL (1. 7) modulation, recording pits Pt having its run length limited in a range from 2T to 8T are recorded on the recording track.
The optical pickup 1 supplies the comparator 4 with a read signal "a" having a signal level as shown in FIG. 2 in accordance with the reading of the read recording pit Pt formed on the optical disc 3. The comparator 4 compares the signal level of the read signal "a" with the level of a predetermined threshold "b". In this event, the comparator supplies a binary signal c at high level, i.e., "1" to each of a determining circuit 5 and a clock generator circuit 6, when the signal level of the read signal "a" is higher than the level of the predetermined threshold "b", and supplies the binary signal "c" at low level, i.e., "0" to each of the determining circuit 5 and the clock generator circuit 6, when the signal level of the read signal "a" is lower than the level of the predetermined threshold "b".
The clock generator circuit 6 generates a reproduced clock signal "d", the phase of which is synchronized with the edge timing of the binary signal "c". The determining circuit 6 samples the binary signal "c" supplied from the comparator 4 at the timing of the reproduced clock signal "d", and outputs sampled values as a reproduced digital signal "e".
As described above, the digital signal reproducing apparatus shown in FIG. 1 is configured such that a read signal is binary-coded using a predetermined threshold, a reproducec clock signal having the phase synchronized with the edge of the binary signal is generated, and the binary signal is synchronized with the reproduced clock signal to produce a reproduced digital signal.
In this configuration, the higher the density of the recording pits Pt recorded as the digital recording signal, the lower the amplitude level of the read signal, so that the level change of the read signal exhibits a less steep slope. Particularly, the shorter the run length NT of the recording pits PT, the more gradulal the slope of the level change of the read signal.
For example, with a recorded signal encoded in conformity with the RLL (1, 7) encoding, a read signal produced in accordance with the recording pit Pt having the shortest run length 2T has an amplitude level lower than the amplitude level of a read signal produced in accordance with the recording pit having the longest run length 8T, as shown in FIG. 3. Also, as indicated by a broken line in FIG. 3 the higher the frequency of a read signal conforming to higher density recording, the lower the amplitude level.
Furthermore, as shown in FIG. 4, a read signal produced in accordance with a recording pit Pt having a run length 2T indicated by a solid line presents a level change of a more gentle slope than a read signal produced in accordance with a recording pit Pt having a run length 8T indicated by a broken line. This is because the amplitude level of the read signal becomes lower as the density of recording increases. This is also because the read signal produced in accordance with the recording pit Pt having the run length 8T has a waveform resembling a square wave owing to harmonic components included therein, whereas the read signal produced in accordance with the recording pit Pt having the run length 2T has a sinusoidal waveform since it does not include harmonic components.
In the digital signal reproducing apparatus as shown in FIG. 1, if the read signal "a" is binary-coded using the predetermined threshold "b", larger jitter are likely to occur in the reproduced clock signal "d" as the read signal a presents a level change of a more gradual slope.
Thus, the prior art digital signal reproducing apparatus as described above has a problem in that since large jitter may occur in the reproduced clock signal "d" generated based on the read signal corresponding to the recorded signal or recording pit having the run length 2T, this causes an increase in an error rate of a digital signal reproduced in synchronism with the reproduced clock signal "d".